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14 Answers
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You can get started now with a free eBook titled 'Mastering PowerShell'. The eBook was written by long time Windows PowerShell MVP Dr. Tobias Weltner and can be found on www.powershell.com in HTML or as a downloadable .pdf.

Look for an updated version of Mastering PowerShell on www.PowerShell.com very soon. The updated eBook will include Windows PowerShell v2 information and will be free to all members of www.powershell.com (membership to the community site is also free).

Book-wise, I recommend both Windows PowerShell in Action by Bruce Payette and Windows PowerShell Cookbook by Lee Holmes. Payette's book is more of a deep dive into how PowerShell works, and the "Cookbook" is all about solving specific problems.